Why Haven't XEmacs and GNU Emacs Merged?

Richard Stallman writes:
XEmacs is GNU software because it's a modified version of a GNU
program. And it is GNU software because the FSF is the
copyright holder for most of it, and therefore the legal
responsibility for protecting its free status falls on us whether we
want it or not. This is why the term "GNU XEmacs" is legitimate.
But in another sense it is not GNU software, because we can't use
XEmacs in the GNU system: using it would mean paying a price
in terms of our ability to enforce the GPL. Some of the people who
have worked on XEmacs have not provided, and have not asked
other contributors to provide, the legal papers to help us enforce
the GPL. I have managed to get legal papers for some parts
myself, but most of the XEmacs developers have not helped me
get them.
XEmacs was possible because free software means that anyone
can change it and distribute a modified version. I have no regrets
about establishing this freedom for Emacs. Everyone should have
the freedom to change any program, and this is not limited to
changes that the original author likes.
Many people have taken advantage of the freedom to change
GNU Emacs, over the last decade. Most of them were willing to
cooperate on integrating their changes into Emacs. XEmacs arose
as a separate forked version because some of the
developers--starting with Zawinski--were unwilling to do that.
People should have the freedom to decide what to work on,
including the freedom to compete with the GNU project, but it's a
shame when they make that choice. The whole community loses
when someone chooses competition rather than cooperation.
But this is worse than competition--it is unfair competition. The
XEmacs developers can and do copy code they like from Emacs. If
I could copy the code I like from XEmacs in the same way, at
least the rivalry would be fair. But I can't do that't, because
substantial parts of XEmacs don't have legal papers, or don't
have known authors.
As long as we cannot use XEmacs in the GNU system, the GNU
project has to make sure that Emacs is not left behind. In other
words, we have to behave like rivals too, even though we wish
there were no rivalry. When XEmacs developers try to persuade
people to use, test, fix and enhance XEmacs instead of Emacs, the
GNU project can't sit still; we need them to use, test, fix and
enhance Emacs instead.
There is good code in XEmacs, which I'd be happy to have in a
merged Emacs any day. But I cannot copy it out of XEmacs
myself because of the uncertain authorship and/or lack of legal
papers.
This problem could probably be resolved, at least for large parts
of XEmacs, with substantial help from the authors of that code.
Otherwise, the GNU project has to write or find replacements for
it.
I invite people who like Emacs, and want the best possible
version of Emacs to be available for use in the GNU system, to
help in one way or the other.